Severer and severer fuel economy standards have been recently made on automobiles to solve or mitigate environmental issues, and the need to enable low fuel consumption of automobiles has become urgent. Steels for use in automobiles should be developed so as to have higher strengths to meet strong demands for the weight reduction of automobile bodies and should have higher and higher strengths to meet severer and severer fuel economy standards to be set in future. Independently, demands have been made to develop high-strength and low-cost steels, due to stiffer market competition with development of emerging nations.
Exemplary springs for use in automobiles include valve springs used mainly in engines; and suspension springs used for cushioning vibrations transmitted from tires. For example, a valve spring may be manufactured by the following method. Initially, a steel ingot, which has been refined and bloomed so as to have a predetermined chemical composition, is hot-rolled into a round rod having a diameter of about 5.5 to about 8.0 mm, coiled into a coil, and cooled. The cooled coil is subjected to softening anneal at a temperature of around 700° C., and then subjected to a surface-shaving process to remove a decarburized region of the surface layer (this process is hereinafter also referred to as “SV process”). The resulting wire rod is subjected to a heat treatment (also called “patenting”) for improved workability, in which the wire rod is heated to 900° C. or above to be once austenitized, and then immersed in a coolant such as a lead bath or salt bath held to a temperature of about 600° C. to allow the wire rod to undergo isothermal transformation. After having a dense pearlite structure by the action of the heat treatment, the wire rod is drawn to a desired diameter (a diameter of about 3 to 4 mm in the case of a valve spring). The resulting article is subjected to a quenching-tempering treatment for improved spring properties and then processed into a spring shape.
The heat treatment for inducing isothermal transformation has been believed to be necessary for preventing manufacturing troubles such as a break in the wire during processing. However, such heat treatments act as a bottleneck in manufacture and adversely affect the productivity. In particular, steels may have decreasing workability with increasing strengths, and to cover for this, heat treatments for improved workability are tend to be performed for longer durations. This is a significant cause to increase the cost of steel wire rods for high-strength springs. Among such heat treatments, the patenting may require several tens of hours to treat one 2-ton coil. For this reason, simplification of the heat treatment (e.g., performing the heat treatment for a shorter time) or complete omission of the heat treatment will give significant merit to manufacture.
In addition, the heat treatments naturally act as a CO2 emission source. Among such heat treatments, the lead patenting using harmful lead places a large load on the environment. Specifically, demands are currently made to provide a steel wire rod for high-strength springs “having satisfactory workability even when the heat treatment is omitted or a simplified short-time heat treatment is employed instead,” because omission or simplification of the heat treatment, when achieved, may hold promise of significant improvements in productivity, cost reduction, and reduction of load on the environment.
Some techniques of regulating or optimizing hot roll conditions have been proposed so as to improve workability of steel wire rods for springs. As used herein the term “workability” also refers to and includes break frequency (burnout rate) and die life in the surface-shaving process (SV process) and wire-drawing process which are working steps performed between rolling and quenching-tempering treatment (workability in the SV process is hereinafter also particularly referred to as “SV workability”).
In relation to these techniques, Patent Literature (PTL) 1, for example, discloses manufacturing of a wire rod by hot-rolling a work at a heating temperature of 1000° C. or below, in which finish rolling is performed at a temperature of 1000° C. or below; forcedly cooling the work down to a temperature of 650° C. to 750° C.; coiling the work into a coil; and cooling the coil down to 600° C. at a cooling rate of 1° C. to 10° C. per second to give a wire rod. The resulting wire rod has a reduction of area of 40% or more and exhibits good wire drawability even though a heat treatment is omitted.
This technique is intended to suppress the generation of a supercooling structure and to obtain a fine pearlite structure. However, the presence of such a fine pearlite structure, when employed alone, is insufficient for improved workability of a high-strength steel wire rod having a tensile strength of 1050 MPa or more. With a reducing grain size of such fine perlite structure, a high-strength steel wire rod may disadvantageously have an increasing hardness, have inferior wire drawability, and become susceptible to a break contrarily. The technique employs forced cooling down to a temperature of 650° C. to 750° C. before coiling. However, if this process is applied to a steel wire rod for high-strength springs, the resulting steel wire rod may highly possibly have a larger deformation resistance to cause laying failure.
Independently, PTL 2 proposes a technique of densely coiling a wire rod after finish rolling into a coil so that the coil ring pitch is one tenth or less the ring diameter; and slowly cooling the coil. This technique is intended to reduce the hardness of the rolled rod and to enable a SV process of the as-rolled rod. This technique, however, may fail to give a steel wire rod having satisfactory workability, because grains become more and more coarse during slow cooling and have a larger variation in grain size, although the structure has a lower hardness. In addition, decarburization increasingly occurs during slow cooling to cause the product spring to have inferior quality.